1. Field of Invention
This invention pertains to a point to multipoint radio access system. The invention particularly pertains to the configuration of such system which utilizes a number of different modulation methods, the modulation methods being distinguishable with regard to the bandwidth efficiency but also with regard to the coverage range and the interference immunity.
2. Related Art and Other Considerations
Radio access methods have become frequently used to enable the rapid and economic implementation of the access network in modern telecommunication networks. Most known solutions up to date are based on conventional mobile cellular techniques, where the subscribers are fixed instead of mobile. One disadvantage of using cellular techniques for the access network is that the radio spectrum is limited and the implication of this is that these systems normally have a too limited capacity, especially in cities.
Recently a new point to multi point system has been suggested that uses microwave links between a central base station and subscriber terminals. The central base station uses normally 45 or 90 degree sector antennas and the terminals directive antennas pointing towards the base station.
In the suggested system, frequency reuse can be very effective by the use of alternating vertical and horizontal polarizations for the links in adjacent sectors. Also, the line of sight radio links lends themselves to high quality connections, the capacity of which can be adapted individually to different capacities (bitrates).
To further improve on the frequency economy of this system, it has been suggested to use a number of different modulation types for the links. The highest order of modulation requires minimum bandwidth for a given bitrate but has also the minimum range and is most sensitive for interferences from other links. The lowest order of modulation is the most robust. It has the longest coverage range and is also the least sensitive to interferences from other links.
The problem in this kind of system is then to select for each link the most suitable modulation, considering bandwidth economy, coverage range and the foreseen interference situation.
A system of this kind is described in a paper entitled "Link Capacity and Cellular Planning Aspects of a Point to Point Fixed Radio Access System", by A Bollmann, D. Chicon, and M Glauner, European conference on Radio Relay Systems 1996, Bologna, Italy. However heretofore there has not been a clear cut procedure for arriving to a suitable strategy to allocate suitable modulation types and bandwidth to each terminal in the system.
What is needed, therefore, is a system which enables a proper choice of a modulation type considering bandwidth efficiency, coverage range and the interference situation.